Movies are produced in 24 Hz frame frequency and progressive scanning format (denoted 24p) for projection in film theatres, adhering to a worldwide standard for 35 mm film. The projection speed has been fixed since many decades to 24 full pictures per second. Accompanying sound signals, typically present as optical tracks on the same material, are replayed with the same speed. Such playback can be performed in virtually any existing film theatre in the world.
The situation becomes different when such movies are played back via a television system, e.g. by a broadcaster. The major TV systems in the world use interlaced scanning and either 50 Hz field frequency (denoted 50i, e.g. in Europe, China and Australia for PAL and SECAM) or 60 Hz field frequency (denoted 60i, e.g. in USA and Japan for NTSC). In 60i countries the TV system is constructed such that 30 full video frames are displayed per second. Movie playback is performed with basically correct overall speed by presenting one film frame alternately in two or in three video fields using 3:2 pull-down. It should be noted that in the NTSC colour TV system the actual field rate is 59.94 Hz, which means that the movie is actually broadcast with 23.976 frames per second, i.e. slower than original speed by a factor of 1000/1001. This is generally not noticed by TV viewers.
In 50 Hz countries the situation is more difficult. The simplest solution is running the movie with 25 frames per second, i.e. 4.2% faster. Although by this method the sound has a 4.2% higher pitch (nearly one half-tone higher), typically customers do not complain. If movies are recorded on A/V media (e.g. VCR Videos, CDIs and DVDs) and such media are played back using TV receivers as presentation devices, generally the same principles apply. If the accompanying sound present on the medium is in digital form (e.g. linear PCM or AC-3 coded or DTS coded), either the playback device must decode and present the audio information with 4.2% higher speed, which generally is not a problem, or DVD Video or CDI discs sold in 50 Hz countries must contain audio data streams that are already encoded such that, while the video signal is output with 4.2% higher speed, the player's audio decoder automatically outputs the correct speed and pitch of the audio signal.
To summarise, there are currently three established playback speeds for AV streams (Audio Video streams): 24 progressive frames per second (24p) used in cinemas, 50 interlaced fields per second (50i) and 60 interlaced fields per second (60i).
An alternative would be carrying out audio decoding, pitch-conversion and audio re-encoding in the player. However, this would require very expensive player hardware and higher license costs because of the additional audio encoding.
If a disc manufacturer produces a record for more than two different speeds, then a transformation from one speed to one of the other speeds would be the first choice. Because it consumes too much time and money to produce the movie again with the new speed, the original movie will be used again, i.e. the original record will be transcoded to the new speed. Usually moviemakers transcode 24p movies to 60i DVD Videos and to 50i DVD Videos.
Modern TV receivers can handle different playback speeds while keeping the same colour system PAL, NTSC or SECAM. For example, many TV sets sold currently in Europe can present replayed 60i records. But many old TV sets can not handle video input signals having such speed shifts. Even some modern 100 Hz TV sets are malfunctioning when receiving 60i input data. Some old devices play back coloured 60i content without colour, other devices can not play back even a stable picture. For such reason e.g. the DVD Video Format distinguishes between 50i and 60i countries for getting discs with their preferred/supported speed.
Note that the (analog) colour system does not matter anyway because it is not part of the recorded (MPEG) bit streams. The appropriate colour system will be applied or encoded after decompression of the video content.
As mentioned above, 3:2 pull-down is used for transcoding from 24p to 60i. The consumers in 60i countries know and widely accept the resulting drawbacks like slightly jerking video playback due to field repetition. A 4.2% faster playback speed is used for transcoding from 24p to 50i. Most consumers in 50i countries accept the resulting drawbacks like 50i flickering and increased pitch. Some drawbacks are already reduced or even removed, e.g. by using 100 Hz TV sets with sophisticated motion compensation. However, the total playing time is 4.2% shorter than in cinema. The advantage of such speed-up is that the video playback does not have any jerks, but the audio stream must be transcoded to perform the faster playback. So, the audio streams of the same movie differ significantly between 50i discs and 60i discs, but the (MPEG) video stream is very similar. The main difference of the video streams is that the 60i video stream contains additional 3:2 pull-down flags and different time stamps. MPEG means e.g. MPEG-2 Video, MPEG-4 Video or MPEG-4 AVC.
Consumers accept the drawbacks of the transcoding method used in their country but do not like the different drawbacks of the other country. The 60i consumers don't like the flickering of 50i TV sets (theoretically this could be overcome by replacing current 60i TV sets by TV sets, which are also compatible to 50i by applying 100 Hz techniques, or by using slow LCD displays). The 50i consumers don't like the jerks of the 60i video (theoretically this could be overcome by replacing current discs, players and TV sets by 24p compatible discs, players, and TV sets).
FIG. 1 shows at the left side a known DVD player DVDP with a known 50i disc 50iDVD inserted. The player outputs 50i AV. The right side depicts how the 50i disc content is presented on an old-fashion 50i only TV set OLDF as 50i, on a currently produced 50i (and 60i compatible) TV set CUR as 50i, and on a modern 24p (and 50i and 60i compatible) TV set MOD as 50i, which TV sets or displays are connected to player or recorder DVDP.
FIG. 2 shows at the left side a known DVD player DVDP with a known 60i disc 60iDVD inserted. The player outputs 60i AV. The right side depicts how the 60i disc content is presented: no presentation on the old-fashion TV set OLD, on the currently produced TV set CUR it is presented as 60i, and on the modern TV set MOD as 60i, which TV sets or displays are connected to player or recorder DVDP. The only restriction may result from a region code on the disc.
FIG. 3 shows at the left side a known 24p DVD player DVDP24p with a known 24p disc 24pDVD32FL inserted that includes 3:2 pull-down flags. The player outputs 24p or 60i AV. The right side depicts how the 24p or 60i disc content is presented: no presentation on the old-fashion TV set OLD, on the currently produced TV set CUR it is presented as 60i, and on the modern TV set MOD as 24p, which TV sets or displays are connected to player or recorder DVDP24p.
DVD players do not support 24p playback. But if DVD would support 24p playback, then FIG. 4 shows at the left side such a 24p DVD player DVDP24p with a 24p disc 24pDVD inserted that has no 3:2 pull-down flags. The player outputs 24p AV. The right side depicts how the 24p disc content is presented: no presentation on the old-fashion TV set OLD and on the currently produced TV set CUR, but on the modern TV set MOD it is presented as 24p.
EP 04300722.8 describes 24p disc replaying using 3:2 pull-down for 60i presentation and 24:1 pull-down for 50i presentation. However, the video field repetition in 50i presentation can lead to jerks during scenes with strong motions, which was previously unknown to viewers in 50i countries.
PCT/EP2004/012483 describes 24p disc replaying for 50i presentation whereby the disc is replayed such that audio is presented with its original pitch but video fields or frames are inserted adaptively so as to cause minimum visible motion judder.
EP 04006792.8 describes 24p disc replaying for 50i presentation whereby the video is replayed faster by a factor of 25/24 and part of the audio is dropped in order to achieve upon reproduction on average the original audio pitch. The dropping is controlled by a marking information signal. However, such audio frame dropping can lead to stumbles during harmonic audio phases.